Cask-conditioned ale is beer that is naturally carbonated, because the beer is put in the sealed cask, or firkin, before fermentation is complete. The gas produced by the fermentation is then absorbed in the beer, explains Empire head brewer Tim Butler. That produces a gentle, natural carbonation. Carbonation in standard kegged and bottled beer is produced by adding carbon dioxide (CO2), or sometimes nitrogen, after fermentation. Thatís what gives most beer its fizz.

What is Sour Beer?

Unlike modern brewing, which is done in a sterile environment to guard against the intrusion of wild yeast, sour beers are made by allowing wild yeast strains or bacteria into the brew. Traditionally, Belgian brewers allowed wild yeast to enter the brew naturally through the barrels - an unpredictable process that many modern brewers avoid. The most common agents used are Lactobacillus, Brettanomyces, and Pediococcus. Another method for achieving a tart flavor is adding fruit - most commonly cherries (to produce kriek) or raspberries (to produce framboise) - during the aging process, to cause a secondary fermentation. Because of the uncertainty involved in using wild yeast, the sour beer brewing process is extremely unpredictable. The beer takes months to ferment and can take years to mature.